1. Field of the Invention
The present invention relates to a laser light intensity control device including a semiconductor laser that scans a photoconductor by emitting laser beams and forms an electrostatic latent image, a laser light intensity control method, and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses using a semiconductor laser as the light source offers an automatic power control (APC) function for achieving a constant laser light intensity in order to provide high quality images with no density variations. When executing the APC function, the laser light intensity is detected using a method that turns on a laser in a non-printing area in a main scanning direction, converts the laser power into electric current using a photodiode, and monitors a voltage generated in a resistor. Then, the detected laser light intensity is compared with a target laser light intensity preset in the image forming apparatuses. If there is a difference between the detected laser light intensity and the target laser light intensity, the drive current of the semiconductor laser is adjusted to bring the laser light intensity close to the target light intensity.
In the image forming apparatuses using a semiconductor laser, if the semiconductor laser is continuously driven to emit light, the laser light intensity decreases. This phenomenon is called droop and is unique to semiconductor lasers. Droop causes density variations in images, resulting in reduced image quality. In order to reduce droop, some related art image forming apparatuses use a method that supplies a predetermined so-called bias current to a semiconductor laser to keep the semiconductor driving without emitting laser beams and, at the time of image formation, superimposes a current responsive to a modulation signal on the bias current to cause the semiconductor laser to emit light.
The related art image forming apparatuses using this method can provide improved responsiveness for causing the light emission time of the semiconductor laser to respond to the modulation signal which indicates ON time of the semiconductor laser. Japanese Patent Laid-Open Publication No. 60-234389 teaches the effects of bias current on the responsiveness of semiconductor lasers.
In order to achieve a constant laser light intensity, other related art image forming apparatuses use a method that divides a current for driving a semiconductor laser into a light emission current and a bias current which are independent from each other and sets the bias current based on measurements of the slope efficiency. For example, Japanese Patent Laid-Open Publication No. 2002-321402 discloses a method that maintains a constant laser light intensity with a fixed light emission current and a variable bias current using an APC function.
In recent years, the above described image forming apparatuses using a semiconductor laser have been expected to provide higher printing speed and better image quality. To satisfy such a demand, image forming apparatuses having a multibeam light source formed of plural semiconductor lasers have been put into practical use.
The image forming apparatuses having a multibeam light source are required to control the laser light intensity more precisely in order to achieve a constant light intensity of each beam and to equalize the light intensities of the beams.